Conventional gas turbine engines include an integral oil system in which oil is pressurised by a pump driven by one of the rotating shafts. Oil flow from the pump and the system pressure require that the relevant engine shaft be rotating. Hence at the point of engine start the shaft is stationary and there is no pump rotation or delivery pressure. These conditions may result in inadequate oil supply or inadequate pressure in various areas in the lubrication system during the start cycle.
Prior art proposals for the starting process of a gas turbine engine include the use of a pneumatic starter which is fed by air pressure from a supply source external to the engine and under the control of a starter air valve. When the engine reaches a self sustaining speed the starter air supply is switched off.